Liquid crystal displays are widely used for image display devices such as television screens, video cameras, computers and computer displays. Since the liquid crystal display itself does not luminesce, a back light is generally used for illuminating the screen from behind the display to render the screen readily visible. Cold-cathode flat fluorescent lamps are used as such back lights.
The flat fluorescent lamp comprises two parallel glass plates each having a fluorescent coating over the inner surface and defining therebetween a space which is closed along the peripheries of the glass plates to form a discharge chamber, and a pair of discharge electrodes arranged inside the discharge chamber. For use with a liquid crystal display with a large screen, the flat fluorescent lamp also needs to have a large size approximately matching the size of the display screen, so that there arises a need to use glass plates of increased thickness and give the lamp a sufficient strength against pressure. This entails the problem that the flat fluorescent lamp itself becomes large-sized and heavier.
To overcome this problem, ITEJ Technical Report, Vol. 12, No. 15 (issued on Mar. 25, 1988), pp. 49-54, discloses a lamp. U.S. Pat. No. 4,920,298 discloses another lamp, which has the structure shown in FIGS. 13 and 14. This lamp comprises a front glass plate 10, a rear glass plate 12 and a glass frame 42 surrounding a space therebetween. Spacer rods 38 against pressure are provided between the front and rear glass plates 10, 12, whereby the lamp is given a strength to withstand the atmospheric pressure and made resistant to breaking without increasing the thickness of the front and rear glass plates 10, 12.
At the positions where the spacers are provided, however, the inner surface of the front glass plate 10 has no fluorescent layer 32, failing to luminesce, so that the shadows of the spacers are cast on the display screen. To give the luminescent surface of the flat fluorescent lamp luminance of the highest possible uniformity, ingenuity is exercised in diminishing luminance irregularities by forming a fluorescent coating 40 on the side faces of the spacer rods 38 (see FIG. 14) and providing a diffusion panel 22 (see FIG. 13) over the luminescent surface.
However, forming the fluorescent coating on the spacer rods requires additional work and results in an increased cost. Further when the luminescent surface of the flat fluorescent lamp has marked irregularities in luminance, the diffusion panel fails to eliminate the luminance irregularities if provided.